1. Field of the Invention
The invention relates in general to protective relays, and more specifically to protective relays for protecting electrical machines connected to an electrical power system from the effects of harmful subsynchronous oscillations.
2. Description of the Prior Art
Turbine generators which generate electrical power have been severely damaged by shaft torques produced by electrically initiated mechanical oscillations at resonant frequencies of the mechanical system. Damaging oscillations may build slowly due to interaction between the electrical power system and the mechanical system of the turbine generator; or, a transient of high magnitude, such as due to a short circuit on a series-compensated transmission system, may provide a "shock" torque which initiates a subsynchronous oscillation of the mechanical system at a resonant frequency.
Many different approaches have been used to try to solve the problem presented by subsynchronous oscillations, such as by inserting electrical power filters into the electrical line to block or dampen subsynchronous currents. Also, additional control has been provided for series capacitors, which reduces short circuit current magnitudes. Additional control has also been provided in the excitation system of the turbine generator, which increases system damping. These approaches to solving the problem, however, may malfunction or fail. Such failure, for example, may be the result of filter detuning, drift of the electrical power system from the synchronous frequency, or failure of initiating signals to reach the series capacitor control. Also, since the problem is very complex, the "solution" may not protect the system against subsynchronous resonance from all possible causes. Thus, it would be desirable to back any solution to the subsynchronous resonance problem with the application of a protective relay which detects an unacceptable subsynchronous oscillation condition and either provides a trip signal for a circuit breaker to remove the apparatus to be protected, such as a turbine generator, from the line, or it initiates some other form of corrective action.
A protective relay for such a function must distinguish between subsynchronous oscillations which are potentially harmful, and those which are not, as a turbine generator should only be tripped from the line when damage producing oscillation thereof is imminent. However, since such oscillations may be produced in a very short period of time, the processing speed of the relay must be fast enough to provide the detection and corrective action before actual damage occurs. The protective relay must have a wide operating range and a sensitivity which will detect 0.01 PU (per unit) of subsynchronous current in the presence of a synchronous (i.e., 60 hertz) current of many PU. The subsynchronous current may also have a magnitude of many PU.
Copending application Ser. No. 757,177, filed Jan. 6, 1977, in the name of S. C. Sun, entitled "Apparatus for Detecting Subsynchronous Current in Power Systems", which application is assigned to the same assignee as the present application, now U.S. Pat. No. 4,125,884 discloses new and improved monitoring apparatus for providing a signal responsive to subsynchronous current flowing in an electrical power system. A signal responsive to a current in one of the phases of the electrical power system is multiplied by a reference signal having a synchronous frequency and a constant amplitude. The product contains components having frequencies equal to: (1) twice the synchronous frequency, (2) the sum of the synchronous frequency and each subsynchronous frequency, and (3) the difference between the synchronous frequency and each subsynchronous frequency. A wide band or bandpass filter passes component (3) which relates directly to subsynchronous oscillation frequencies, while substantially blocking or attenuating components (1) and (2).
Our co-pending application Ser. No. 757,178, filed Jan. 6, 1977, entitled "Apparatus for Protection Against Subsynchronous Currents in a Power System", which application is also assigned to the same assignee as the present application, now U.S. Pat. No. 4,106,071 discloses detection apparatus for processing a signal responsive to subsynchronous current in an electrical power system, such as the signal (3) hereinbefore referred to. This detection apparatus provides a trip signal in response to a growing oscillation in the frequency range of interest, such as 15 to 45 hertz, when the oscillation exceeds a predetermined threshold magnitude, and a predetermined magnitude at the end of a predetermined period of time.